A process to selectively monoepoxidize dienes is disclosed in U.S. Pat. No. 4,897,498. The diene of most commercial interest to epoxidize is butadiene. The resulting desirable product is 3,4-epoxy-1-butene (also known as butadiene monoepoxide, epoxybutene or EpB.TM. oxirane).
The catalysts employed in the reaction are promoted, supported silver containing catalysts. The promoters may be alkali metal halides. One example of a such catalyst is an Al.sub.2 O.sub.3 supported, CsCl-promoted Ag catalyst. Under standard epoxidation conditions, the halide is being continually removed from the surface of the silver and consumed, thus causing the catalyst to become unstable and the reaction to be nonselective. In addition, the complete combustion of C.sub.4 H.sub.6 to CO.sub.2 and H.sub.2 O occurs and the reaction becomes thermally unstable. Excessive production of CO.sub.2 and H.sub.2 O is undesirable because it renders temperature control in the process extremely difficult with thermal run-aways to over 400-500.degree. C. within 8 hours of being on line and valuable feed stock is consumed. Once a thermal run-away occurs, the catalyst becomes irreversibly non-selective. The temperature in the process must be carefully controlled to avoid local over-heating of the catalyst, which results in accelerated promoter loss, greater rates of combustion and possible permanent degradation of the catalyst making it unfit for further use.
This problem is overcome, as disclosed in U.S. Pat. No. 4,950,773, by the carefully controlled addition of organic halides to the reactor feed stream soon after the reactor is brought on line. The halide of the feed stream is continually deposited in a controlled manner on the catalyst surface at a rate equal to the amount of halide being removed from the catalyst such that the catalyst is stabilized. However, this is a delicate balance to maintain. If too much organic halide is deposited the catalyst deactivates; if too little is deposited the catalyst becomes non-selective and unstable.
Thus, there exists a need in the art to have a catalyst for the monoepoxidation of dienes that reduces catalyst degradation, promotes thermal stability, provides increased selectivity, and does not require extensive monitoring. This is especially critical during the first hours and possibly days that a fresh catalyst is exposed to the butadiene plus oxygen-containing feed stream. This period of time, during which a catalyst reaches steady-state catalytic activity, is critical in defining the resultant activity and stability of promoted silver catalysts for the epoxidation of butadiene. Accordingly, it is to the provision of such an improved catalyst for the production of epoxides that the present invention is primarily directed.